Ribonucleic acid (RNA) is available as a raw material for a drug or a flavor enhancer such as 5′-inosinic acid (IMP) or 5′-guanylic acid (GMP). In general, the ribonucleic acid production employs yeast cell bodies that are cultured using waste molasses or liquid sugar as a major carbon source. Especially, yeasts are utilized for this purpose, Candida utilis and Saccharomyces cerevisiae. In an approach to increasing the ribonucleic acid content of yeast, attempts have been made, which include culturing in antibiotic-added media, the examination of major carbon sources and the isolation of mutant strains. Specifically, as U.S. Pat. No. 3,909,352 reports, a yeast of a Candida species is mutated, a strain having KCl susceptibility is then isolated, and a yeast cell body containing a ribonucleic acid content of 12% by weight or more in the solid mass (dried cell body) is thus produced. Further according to the publication of JP-A-11-196, 859, a low temperature sensitive mutant is isolated from Candida utilis and a yeast cell body containing a ribonucleic acid content of 20% by weight or more in the solid mass is thus obtained.
Brewer's yeast that is used in brewing beer is washed after beer production; and its heat-dried cell bodies are utilized for the productions of dietary supplements and yeast extracts, and thus they are widely recognized as safe. Nevertheless, the yeast cell bodies recovered after beer production contain ribonucleic acid as much as at a level of about 4 to 6% in their solid masses; therefore, they have been considered as materials that are inadequate to extract ribonucleic acid on a commercial scale. Hence, studies of increased ribonucleic acid contents have been focused on Candida yeast species rather than on Saccharomyces yeast species, including bakers' yeast. The Candida yeast cell bodies, however, cannot necessarily be said to be non-toxic to the health of humans and animals and adequate precautions will be needed in the utilization of their products as foodstuff or feed additives. For this reason, there was proposed a method for increasing ribonucleic acid in a yeast cell body which uses a baker's yeast (Saccharomyces species of Saccharomyces cerevisiae DSM5616) under limitative fed-batch culture conditions (JP-A-05-176757). This method allowed a cell body having a ribonucleic acid content of 10% or more to be produced even when the solid mass exceeded 30 g/L. However, it was again only possible to attain the high ribonucleic acid content by limiting the strain for use in culturing as described above.
Thus, up till the present time there has not been known any brewer's yeast conventionally used having a ribonucleic acid content of 10% or more based on the weight of its cell body. Nor has there been an accomplished method for increasing the ribonucleic acid content of a brewer's yeast cell body to the aforementioned level.
Accordingly, this invention aims at providing a brewer's yeast cell body containing a high ribonucleic acid content comprising 10% by weight or more of ribonucleic acid based on the weight of the cell body; and it further aims at providing a method for efficiently producing such a brewer's yeast cell body without any restrictions of the strain to be used.